Disposable mercury alloy pattern and method of making the same



1958 I. R. KRAMER 2,857,641:

DISPOSABLE MERCURY ALLOY PATTERN AND METHOD OF MAKING THE SAME Filed Sept. 16, 1955 @JIE... 5'. Z2

INVENTOR. I R. Kramar WM a. nTwnl fys United States Patent DISPOSABLE MERCURY ALLOY PATTERN AND METHOD OF MAKING THE Application September 16, 1955, Serial No. 534,657

15 Claims. (Cl. 22-164) This invention relates to impermanent or disposable casting patterns suitable for forming shell molds for precision castings, to the preparation of such casting patterns and to the preparation of shell molds with such patterns, although some aspects of the invention are of a broader scope.

lmpermanent patterns formed of pure frozen or solid mercury as disclosed in Kohl Patent 2,400,831 have been found of great value in the precision casting art because they make it possible to produce thin-walled porous shell molds with mold cavities of fine surface finish for use in casting metallic objects having intricate shapes which heretofore could not be cast with precision molds made with patterns other than frozen mercury patterns. However, the forming of thin shell molds over casting patterns of frozen or solid pure mercury must be carried on at very low temperatures of 60 C. to 75 C. and requires complicated procedures.

Many unsuccessful efiorts have been made in the past to find a substitute for casting patterns of frozen solid pure mercury that would have a freezing temperature higher than 40 C. q

The present invention is based on the discovery that casting patterns or casting pattern sections formed of the intermetallic compound of Hg Tl or of a mixture of Hg Tl with either one of the intermetallic compounds of Hg Tl or Hg Tl which freeze near 16 C. may be used with great advantage for producing thin porous shell molds which heretofore could be produced only with casting patterns of solid or frozen pure mercury.

The foregoing and other objects of the invention will be best understood from the following eiremplifications thereof, reference being had to the accompanying drawings wherein;

Fig. 1 is an elevational view of an impermanent pattern of a hollow gas turbine bucket made of the metal compound of the invention;

Fig. 2 is a bottom view of the bucket of Fig. 1;

Fig. 3 is a cross-sectional view along lines 33 of Fig. l;

Fig. 4 is a vertical cross-sectional view of the bucket of Fig. 1; and V Fig. 5 is a side view of the bucket of Fig. l as seen from the right.

It has long been recognized that the use of casting patterns of pure frozen mercury for making thin porous shell molds requires complicated equipment and procedures. The frozen mercury pattern has to be formed in special dies which must be handled in a special way in order to prevent formation of ice when cooled to the low temperature of 60 to -75 C. to freeze the pure mercury into the self-supporting casting pattern. Special refrigerating equipment has to be provided for freezing the mercury in the die and the freezing of the mercury within the dies has to be carried on in a very special way. The liquid investment slurry for forming a shell mold around the frozen mercury requires a liquid carrier of unique properties including the property of maintaining ICC the refractory particles and the binder ingredients uniformly dispersed within the liquid carrier at low temperatures of 60 to C., and the further requirements that it should be impossible to readily drive off or evaporate the carrier from the shell mold at temperatures of 60 to 75 C. at which temperature the frozenmercury pattern remains strong and does not sag. When using frozen mercury patterns for making large size shell molds, the frozen mercury pattern and the shell forming operation must be carried out at still lower temperatures, such as to C. Thus far, for best results, it was found necessary to use the costly monochlorodifluoromethane (Freon 22) as the liquid carrier and a good deal of this costly carrier is lost in each operation of forming the shell mold on a frozen mercury pattern.

The very low temperatures of 60 to -75 C. at which they have to be performed slows down the work of forming the shell mold, requires special cumbersome techniques, and elaborate refrigerating equipment for maintaining all facilities used in making theshell mold at the low temperatures of 60 to 75 C.

Because of these elaborate and special facilities and techniques required for producing shell molds with frozen mercury patterns, the process has been heretofore limited to the production of either specially shaped or largesize castings which could not be produced by the lost wax or other known precision casting methods.

Notwithstand these limitations and complications, casting patterns of frozen mercury have been found to be of great value in the precision casting field because in addition to their desirable features referred to above, pure mercry has the special characteristics which are of great importance for a casting pattern material.

In order to be suitable for making molds, it is essential that the material of the casting pattern should be easily removable from the-mold cavity with the further critical requirement that when the mold is heated to an elevated temperature, such as 800 to 1000 C., for rendering the raised temperature binder effective, traces of the casting pattern material left in a crevice 0f the mold shall not react with the refractory material of the mold in a way that will impair the surface of the mold cavity.

For example, traces of lead or tin left in the cavity of a mold, will upon heating of the mold to an elevated temperature form a corresponding metal oxide which reacts with the refractory mold ingredients, resulting in crust formations along the mold cavity and requiring in most cases scrapping of the mold. In cases of other metals the oxides of which are volatile, these metals react with the refractory particles of the mold before evaporation of the metal or before completion of its evaporation, likewise resulting in impairment of the mold cavity and scrapping of the mold. Pure solidified mercury meets this critical requirement of casting patterns, because it boils or volatilizes at about 356 C., so that when the shell mold formed on such casting pattern is heated to elevated temperatures of 800 to 1000" C., any traces of pure mercury of the pattern left in a crevice of the mold will volatilize without impairing the surface of the mold cavity.

Another important advantage of pure mercury for casting patterns is the fact that it makes possible to form out of it a plurality of complementary pattern sections which when brought into contact abutment along their mating surfaces, become integrally united and welded into the desired self supporting complex casting pattern which could not be produced by freezing or solidifying mol en mercury in a cavity of a correspondingly shaped master die. Such uniting or welding of complementary casting pattern sections into a desired composite final casting pattern is known in the casting art as booking.

The very desirable feature of contact booking of complementary pattern sections of pure solid mercury is not acquire surface films which tend to obstruct or int pede the booking of complementary pattern sections.

On the other hand, the process of forming shellmolds over casting patterns of solid pure mercury must be carried on at very low'temperatu'res of 60 to 75 'C.

at which the frozen pure mercury exhibits substantial strength and the forming of the shell molds at such low temperatures requires complicated equipment and procedures.

The present invention is based on the'discovery that there exists one other metal substance, namely the intermetallic or chemical compound Hg Tl either alone or in solid solution or mixed with either one or the other of the two closely'related intermetallic compounds Eg and Hg Tl, which freezes near 16' C. and may be used for forming casting patterns having shiny bright exterior surfaces and the other unique properties which make it possible to use it as casting patterns in formingthin shell molds, while avoiding the complicated equipment and procedures required when using casting patterns ofpure solid mercury.

The intermetallic or chemical compound Hg Tl has a face centered cubic crystal structure. It freezes at 16.45 C., a factor very desirable for casting patterns. On heating, the intermetallic compound Hg Tl decomposes by syntectic reaction at 15.38 C. into two liquid phases which become homogeneous at temperatures of 16.45 C. and higher, the immiscib'ility gap being about 1 C. The intermetallic compound Hg Tl forms solid solutions with the intermetallic compound Hg Tl as long as the thallium content of the metal body is at least 25.34%. The intermetallic compound-Hg Tl forms a solid solution with the intermetallic compound Hg Ti as long as the thallium content of the metal body does not exceed 33.75%. Low melting eutectics of Hg and Tl are encountered only with thallium contents smaller than 25.34% and greater than 33.75%.

The closely related intermetallic compound Hg Tl and Hg Tl have similar desirable properties. The intermetallic compound Hg Tl has a melting range between about 14 C. and 13.31 C., the melting gap being about 051 C. The intermetallic compound Hg Tl has a melting range between 13 C. and 11.5 C., the melting gap being about 1.5 C.

Although a search through the scientific literature including the book of Hansen Der Aufbau der Zwcistofilegierungen published 1936 shows reports of purported other intermetallic compounds of mercury with metals other than thallium having the desired melting or freezing temperature in the range between about and 50 C., a careful investigation established that-except for the mercury-alkali metal and mercury-alkalirearth metal. compounds which are unusable 1 because unstable .in .the presence of moisturetheso reported compounds .do not actually exist and that the so reported compositions of mercury with other metals do-not have the properties which they would have exhibited had they beentr' e intermetallic compounds. In particular, it has beenfound that reported alleged intermetallic compoundsof mercury with Cd, Pb, Sn, Sr, Sb, Cn do not exist and that compositions corresponding to such reported alleged compounds have such high rate of oxidation, as to exclude the possibility of being used for casting patternsfor forming molds.

For the intermetallic compound Hg TI the volumetric expansion on melting or contraction on solidification is 1.482% by volume, being thus considerably smaller than that of pure mercury which is 3.75%, a factor of great practical importance in forming mold-cavities. At 0 C. the intermetallic compound Hg Tl has a strengthatleasttwice that of frozen pure mercury at --65 to -75C. and.

. for pure mercury. centimeter against air at 20 C. is estimated to be 640 '4 the creep strength of Hg Tl is likewise materially greater. As a result, casting patterns of the intermetallic com pound Hg Tl exhibit much higher resistance to sagging and may be made in much larger dimensions than casting patterns of solid pure mercury without danger of sagging while used in making molds. Its density at 20 C. is estimated to be 12.91 grams per cc. compared to 13.546 Its surface tension in dynes per compared to 487 for pure mercury. Its thermal capacity in calories per gram per degree C. is estimated to be 0.0385 compared to 0.0334 for pure mercury. The partial vapor pressure of the mercury ingredient of the compound Hg Tl is about half that of pure mercury. At room temperature the vapor pressure of the thallium ingredient of the compound Hg Tl is negligible, being 6X 10- mm. of mercury column. Accordingly the hazard due to mercury vapors is diminished by about 50%, the thallium vapor being obviously no hazard.

Each of the-two compounds Hg Tl and Hg Tl has at 0 C. a strength at least twice as great as that of solidified pure mercury at 65 to '0 They also are similar to Hg Tl in their other properties which make them uniquely suitable as casting patterns for use in making thin porous precision casting shell molds.

In practice, very good results are obtained with casting patterns consisting of a solid solution of the intermetallic compound Hg Tl with either one of the intermetallic compounds Hg Tl or Hg Tl wherein the thallium content of the solid solution differs from the stoichiometric proportion corresponding to Hg Tl by about 1% to 1 /2%. (Throughout the specification and claims all proportions are by weight, unless otherwise specified.) However, satisfactory results are also obtained with casting patterns formed of a solid solution of Hg Tl with Hg Tl as long as the thallium content of the solid solution is at least that corresponding to Hg Tl and with casting patterns formed of a solid solution of I-Ig Tl with'Hg Tl as long as the thallium content of the solid solution does not exceed that corresponding to Hg 'l'l. The casting patterns should be substantially free of mercury and thallium, which are not part of the foregoing mercury-thallium compounds.

ThalIium oxidizes in air. When exposedfor a prolonged period to air, the surface of a solid pattern of Hg Tl is slowly oxidized, the thallium in the solid compound Hg Tl forming thallous oxide (T1 0) which melts at 300 C. and vaporizes appreciably at this temperature. Thalous oxide dissolves in water to form thallous hydroxide Tl(OI-l). Thallous oxide may be oxidized to thallic oxide T1 0 or reduced to thallium metal. The casting pattern compound Hg Tl does not react with oxygen-free water. Water which is exposed to air usually contains oxygen absorbed from the air. When the casting pattern compound Hg Tl is immersed in oxygen-containing water. an undesirable surface stratum consisting of thallous hydroxide Tl(OH) is formed thereon. In order i to permit ready book .or uniting between complementary casting pattern sections of Hg Tl the surfaces of the casting pattern sections should be free from metallic ions and other foreign surface strata, and they should have a shiny bright surface comparable in brightness to the exterior of solid pure mercury.

According. to the invention, the casting pattern material consisting of Hg Tl without or with an admixture of either Hg Tl or Hg Tl, is prepared under conditions which prevent or suppress oxidation of the pattern material or of the thallium ingredient thereof as they are combined to form Hg Tl This may be done by reacting the mercury and thallium in proportions corresponding to the compound Hg Tl in an atmosphere ofpurified hydrogen or of purified carbondioxide at 250 C. In practice it'has been found of advantage to combine mercury with thalliumtinto thecompoundHgJlunder aprotective liquid which prevents or suppresses oxidation of the ingredients and also of the formed compound Hg Tl Unless otherwise specified. whatever is stated herein throughout the specification with respect to Hg Tl applies also to solid solutions thereof with either Tg Tl or Hg Tl.

The protective liquid for protecting the pattern material of the compound Hg Tl as it is being formed and also to protect it thereafter against oxidation when exposed to oxidizing media for a long period of time must meet a number of requirements. The protective liquid should be non-reactive and should not dissolve the compound Hg Tl and its ingredients. It should possess a suificiently low freezing point as to permit holding therein the solidified casting patterns of Hg Tl It should have a low effective oxygen transfer from the atmosphere to which it is exposed, such as air, to the compound Hg Tl held under the liquid. The protective liquid must have sufliciently low viscosity so as not to interfere with displacement of the protective liquid from the cavity of the die wherein the casting pattern is frozen into the desired shape. The protective liquid must also minimize and suppress oxidation of the surface of casting pattern sections of Hg Tl during booking into a desired complex casting pattern.

Suitable protective liquids for the compound Hg Tl without or mixed with the related compounds Hg Tl and Hg Tl, and for casting patterns of such compound, are diatomic and polyatomic alcohols of the aliphatic series which are liquid at least between C. and 30 C., and which, in their commercial form which usually contains water, will dissolve thallium oxides. Protective liquids with which good results are obtained are the glycols, such as ethylene glycol and propylene glycol, and the glycerols such as glycerin.

Glycerin (glycerol) is very effective as a protective liquid, but where its viscosity is objectionable it must be mixed with water to reduce its viscosity. Distilled or deionized or, in general, ion-free water may also be used as a protective liquid. It is also desirable that the protective liquid should contain an oxygen binding or suppressing substance or anti-oxident, such as beta naphthol C I-LOH, hydroquinol C H O metadihydroxy benzene C H (OH) or other analogous anti-oxidants.

According to the invention, the protective liquid, such as ethylene glycol, is also used for holding or storing thereunder the liquid metal of the compound Hg Tl preparatory to and during freezing of the compound into the desired casting pattern or casting pattern sections, and also for booking or uniting complementary casting pattern sections into the more complex casting pattern, and for storing therein the completed casting patterns of the compound Hg Tl until removed therefrom for forming the desired shell molds. For sake of simplicity, ethylene glycol will be hereinafter specifically referred to as the protective liquid, it being understood that other protective liquids such as described above may be used instead.

In practice, it has been found'that after the liquid protective bath, such as ethylene glycol, has been used for some time, depending on the extent of its use, when a casting pattern of Hg Tl is held in such long used protective liquid, the casting pattern acquires a dull surface coating or stratum. The dull surface stratum so formed on the casting pattern of Hg Tl is believed to consist of a thallium compound which may be a thallium oxide and which interferes with the uniting or booking of several complementary pattern sections of such intermetallic compound into a desired complex casting pattern over which the shell mold is to be formed. Where the protective bath consists of a liquid, such as ethylene glycol, which is hygroscopic and absorbs water from the air, the water of such bath will dissolve some of the thallium surface compound which was formed on the surface of Hg Tl casting pattern.

The presence of thallium compounds, or in general metallic compounds or surface strata other than Hg Tl 6 l or Hg Tl or Hg Tl on the surface of casting patterns of Hg Tltends to weaken or make diflicult booking of complementary casting pattern sections. It has been found that booking of complementary pattern sections of Hg Tl becomes more difiicult the longer the age of the protective bath in which the casting pattern is held. Furthermore, poor booking was observed at temperatures below 5 C. if the casting pattern of Hg Tl was left under a liquid bath of ethylene glycol for even a short time, such as 4 hours.

One phase of the present invention is based on the discovery that casting patterns or casting pattern sections formed of the intermetallic compound of Hg Tl or of a mixture of Hg Tl with either of the intermetallic compounds of Hg 'l'l or Hg Tl, may be prevented from acquiring a dull surface stratum and be maintained with a shiny or bright exterior surface comparable visually in brightness with that of solidified pure mercury for long periods of time after forming or shaping such casting patterns, provided such casting pattern and casting pattern sections are held in a bath of protective liquid which is free of metal ions and particularly free of thallium ions.

According to the invention, the formation of undesirable or foreign surface strata on Hg Tl casting patterns held in a protective liquid bath for instance of an aliphatic alcohol, such as ethylene glycol which is exposed to air and contains dissolved water or ion-free water which contains dissolved oxygen, is prevented by subjecting the liquid protective bath to an ion-exchange action which suppresses or removes from the bath the thallium ions and the other metallic ion impurities, such as sodium, potassium, calcium, lead, magnesium, copper, etc., which may get into the bath from corrosion, from the air and from other contamination sources.

A practical way for preparing the compound Hg Tl under a protective liquid will now be described. A clean reaction vessel of material that will not combine with mercury, such as stainless steel, is used, and the protective liquid, such as ethylene glycol, is placed therein. A measured amount of pure liquid mercury is then introduced into the reaction vessel under the cover of the protective liquid, and a measured amount of solid thallium in an amount corresponding to the stoichiometric compo-.

sition Hg Tl is placed in contact with the liquid mercury under the protective liquid bath. Good results are obtained if the initial amounts of Hg and Tl contains up to 1%.% Tl in excess of the stoichiometric amount corresponding to Hg Tl resulting in a casting pattern material consisting of Hg Tl containing Hg Tl in solid solution. Good results are obtained with the reaction conducted at room temperature at about 25 C. or in general between 18 C. and 50 C. To facilitate solution of the solid thallium, the contents of the vessel may be stirred with a suitable paddle or may be left to stand until the thallium is completely dissolved and the desired intermetallic compound or compounds are formed. The bottom region of the reaction vessel may be provided with a valve through which the liquid pattern compound may be withdrawn. It is good practice to filter the liquid mercury thallium compound before forming therefrom the desired casting pattern. Satisfactory results are obtained if the liquid compound material withdrawn from the reaction vessel is purified by passing it through a column of distilled or deionized purified water, about five feet long, and near the bottom of which is located a removable filter structure, such as a filter of closely woven cotton, or other filter fabric through which the liquid metal passes into a storing vessel. In the storing vessel the filtered liquid metal compound is collected under the cover of a bath of protective liquid, such as ethylene glycol. All vessels or containers in which the liquid metal compound is treated should be of corrosion resisting material, such as stainless steel, which does not combine or react with mercury.

The just described step of filtering of the liquid casting metal compound shouldalso be applied thereto .after it is. melted out and removed from the mold formed over a casting pattern of the metalcompound, since the pattern metal picks up refractory dust particles fromthe mold and maybesubjected to surface oxidation to some extent.

As explained before, the protective liquid is subjected to an ion exchange actionwhich-suppresses the formation or removes from thetprotective liquid bath thallium ions and other metallic ion impurities present therein. This may be done .by exchanging the metallic ion impurities which may form or be present in the protective bath with the cations of an ion'exchange substance. Any of the known cation exchange substances may be used for such cation exchange action, the commercially available cation exchange resins being most eifective. As an example, any .of the known commercial sulfonic cation exchange-resins or carboxylic-acid cation exchange resins may be used, .of 'which those described in U. S. Patents 2,500,149, 2,466,675and 2,366,007 are typical. As an example, in actual-practice of the invention, the strongly acidic Amberlite IR-120 cation exchange resin having the active group SO H is being used for such cation exchange action. In such cation exchange action, the thallium ions (or other metal ion impurities) of the protective liquid are absorbed by the cation exchange resin which releases the hydrogen ions. The released hydrogen ions combine with hydroxyl ions present in the bath to form water, as the protective liquid is kept free from thallium and other metal impurities. The thallium which is absorbedby the cation exchange resin may be recovered by subjecting the resin to conventional regenerating treatments, with dilute nitric acid, for instance, from which the thallium is then separated, as by electrolysis.

The protective liquid bath of the type described above may also contain, inaddition to the cationic impurities, other impurities which are .anionic in nature, such as sulfites, sulfates, carbonates, or decomposition products of ingredients of the protective liquid, such as glycolates, in the case of ethylene glycol. Although, under normal conditions, such anion impurities will not cause any material disturbance to the Hg Tl compound held in the protective bath, it is of advantage to keep the protective bath free from anion impurities. To this end, the protective liquid is subjected to an ion exchange action of an anion exchange substance. Any of the known anion exchange substances may be used for such an ion exchange action, the commercially available anion exchange resins being most effective. As an example, any of the known commercial quaternary ammonium anion ex change resins or amine anion exchange resins may.be used, of which those described in U. S. Patents 2,614,099, 2,591,573 and 2,591,574 are typical. As an example, in actual practice of the invention, Amberlite IRA-400 anion exchange resinhaving a strongly basic quaternary amine active group or Amberlite IR-45 anion exchange resin having a weakly basic polyamine active group is effective in subjecting the protective bath to the desired anion exchange action. Such anion exchange resin will absorb from the protective liquid anionic impurities, such as sulphuric acid or glycolates present therein.

It is good practice to provide a cover of protective liquid so purified by ion exchange action to the pattern metal compound Hg Tl (without or with additions of either Hg Tl or Hg Tl) in all operating stages in which it is used and also for storing it Whether in the liquid or in the solid state.

As stated above, diatomic or polyatomic alcohols of the aliphatic series, such as ethylene glycol are used in practice as the protective liquid cover for the pattern metal compound. However, satisfactory results will also be obtained with a protective liquid of distilled or deionized purified water, which is kept free from metallic ionimpurities' by ion exchange action of cation and anion exchangeresins, and which is also kept free of dissolved oxygen by oxygen-inhibitors or antioxidants. In order to depress the freezing point of such protective bath of purified water, ethylene-glycol or a similar glycol compound may be added thereto. Oxygen inhibitors are also of value for keeping ethylene glycol free of oxygen. Any of the known weak organic reducing agents may be used for such oxygen inhibitors. As an example, 1% B- naphthol or 1% triethanol amine in ethylene glycol or ethylene glycol monoethyl ether or other similar glycol compounds will remove therefrom all free oxygen or oxidizing content. In. practice, such inhibitors may be placed in the liquid bath wherein liquid Hg Tl or solid casting'patterns thereof 'arekept stored for a substantial period.

As explained above, it is'essential that oxidation inhibitors be embodied or.dissolved in the protective bath when it consists ofpure'distillcd or deionized water. It is also of great: practical importance that such oxidation inhibitors or antioxidants be embodied or dissolved in the other types of protective-liquids under the cover of which the castingpattern compound Hg Tl is produced or processed into a casting pattern .or stored in any of the stages of the production of casting patterns or of molds made with such casting patterns.

Any of the known oxidation inhibitors or antioxidants which will dissolve in the protective liquid may be embodied therein for this purpose. Among suitable antioxidants are anyof the known weak organic reducing agents, such as triethanolamine, hydroquinol, hydrazine, and its derivatives, such as, phenylhydrazine, alphanaphthol and analogous other antioxidants, a small amount of which, such as /2 to 3% will dissolve in the protective liquid used as a cover for the liquid or solidified Hg Tl compound as it-is being. prepared or stored for forming casting patterns or used for producing molds over'the surfaces of such castingpatterns.

By way of example, if water is used as the protective liquid satisfactory results are obtained by embodying therein hydroquinol. "Good results are obtained by dissolving 1 gram per each milliliter of distilled or purified de-ionized water. The protective liquid solution is made slightly acidby adding per each 100 milliliter of the protective liquid, 12 cubic centimeters (cc.) of dilute 10% acetic acid, the pH of the solution being maintained at about 4, or in general, between 3 to 6. Only acids which can form bufiers with salts of thallium should be used forthis purpose, such as, citric acid, sabinic acid, propionic acid and analogous acids.

By way of another example, in case ethylene glycol or another analogous'protective liquid is used, satisfactory results are obtained by dissolving therein 1% phenylhydrazine to provide for theprotective liquid with an oxidation inhibitor which binds and removes from the water any free oxygen. This'is of great practical importance, since ethylene glycol is strongly hygroscopic and absorbs water vapor from the-air and will hold absorbed therein oxygen.

There will now "be described a specific example as to how a casting pattern of the pattern compound of the invention is prepared. A partible die of a suitable metal, such as stainless steel 'or another metal, the surface of which has been treated to prevent reaction thereof with mercury or thallium, is placed under the cover of a protective liquid in a suitable treating vessel. The protective bath may consist of ethylene glycol containing an equal proportion of water, the liquid bath being kept free from impurities and unbound oxidizing substances by ion exchange resin action and by oxygen inhibitor substances, such as, 1% triethanol amine. The protective bath is maintained at a temperature between about 25 and 40 C., depending on the complexity of the pattern and the corresponding complexity of the die cavity in which the pattern is to be formed. After the cavity of.the partible die heldin the protective bath is 9 filled with the protective liquid and has reached the bath temperature, the liquid Hg Ti compound is poured into the die cavity until it displaces therefrom all the protective liquid and all parts of the die cavity are filled with Hg Tl After the die cavity is filled with the liquid compound Hg Tl it is subjected to a cooling action which freezes the liquid metal compound filling its cavity into the solid casting pattern of the desired shape. This may be done by holding the die which is filled with the liquid metal compound Hg Tl in a freezing bath of the protective liquid, which is cooled to below the freezing temperature of the metal compound Hg Tl such as about C. (Throughout the specification, Where the context permits, the expression pattern compound" means the compound Hg Tl without or with Hg Tl or Hg T1 in solid solution therewith.) The freezing of the liquid pattern compound is carried out in a manner such that a casting pattern which is free from shrinkage and other surface effects is obtained. When the freezing is completed, the die is opened and the frozen casting pattern is removed from the die cavity. The opening of the die and the removal of the pattern may be done under the cover of the protective liquid bath. Alternatively, the die may be removed from the protective bath and opened for removing therefrom the frozen solid casting pattern of Hg Tl whereupon the casting pattern is again placed or stored under the cover of the protective liquid.

Complementary pattern sections of Hg Tl are readily booked, provided the exterior thereof has a shiny bright surface such as exhibited by frozen solid pure mercury. The required shiny surface condition of the casting pattern sections is maintained by keeping the freeze bath free of metallic ions and/ or any oxidizing substances.

In such ion and oxygen free protective bath, booking may readily be accomplished by bringing the mating surfaces of complementary pattern sections into contact without or with the application of external pressure in substantially the same way as is the case of casting patterns of pure solid mercury. Booking of complementary pattern sections may be also effected outside the protective bath since the complementary pattern sections of Hg Tl which have been removed from the purified and oxygen free protective bath will retain their bright shiny surfaces for some time thereafter, in which condition the mating surfaces will unite into the complex pattern on being brought in contact with each other.

In some cases, it has been found desirable to form the complementary pattern sections with mating junction surfaces which have provided thereon a minute thickness of booking stock about 0.0005 inch thick so that booking of the complementary pattern sections may be accomplished by bringing them together along their mating surfaces under limited pressure. Booking with such limited pressure assures that remnants of the liquid films of the protective liquid present will not be entrapped between the mating junction surfaces of casting pattern sections which have been united by booking.

After thus forming the desired casting patterns out of the pattern compound, the completed casting patterns are stored at a temperature such as about 0 C. under a protective liquid bath, which is maintained purified by ion exchange action and by inhibitor substances, such as an addition of 1% of triethanolamine (commercial grade). Such protective liquid bath containing an oxidizing inhibitor such as 1% triethanolamine is also very effective for storing steel dies and preventing rusting or oxidation of their surfaces.

The casting pattern of the pattern compound Hg Tl is removed from the protective liquid for forming thereon the desired shell mold by applying a refractory investment composition to the surfaces of the exposed surfaces of the casting pattern. Before applying the mold forming investment composition to the casting pattern, its surfaces are cleaned from films of protective liquid and any other impurities remaining thereon. in practice good results are obtained by first rinsing the casting pattern with purified and ion and oxygen free water followed with rinsing with a liquid organic solvent, such as, isopropyl alcohol or ethylene glycol monoethyl ether, containing about 1% of an oxygen inhibitor, such as triethanolamine. The casting pattern is invested with the investment composition in an investment location or space which is maintained at a temperature of about 0 C., up to about 8 C.

Shell molds may be formed on casting patterns of Hg Tl with investment compositions of the type disclosed in the application Ser. No. 440,056, filed June 29, 1954, now patent No. 2,790,218, granted April 30, 1957, to Kohl et a1. using as a liquid carrier for the investment composition either a liquid such as methylene chloride or acetone or other solvents having similar properties. Good results are obtained with investment compositions using as a liquid carrier methylene chloride, notwithstanding its relatively high boiling point of 401 C., because the drying of the thin shell mold may be carried on at temperatures up to about 10 C. at which relatively rapid drying of the thin shell mold may be achieved, particularly if there is circulated around the shell mold an atmosphere which is maintained free of methylene chloride vapors.

After forming the shell mold by dipping the casting pattern in the investment composition in a manner such as described in said application Ser. No. 440,056, filed June 29, 1954, the shell mold is dried on the pattern while maintained at a temperature, such as between 0 and 10 C., at which the pattern remains in the solid state. After drying the shell mold, the pattern material is melted and removed from the cavity of the shell mold. The melting of the pattern material may be carried out by first applying a stream of molten pattern material to the exposed parts of the solid pattern followed by application of the molten pattern material to deeper parts of the pattern enclosed in the shell mold. Alternatively, the pattern with the shell mold formed thereover is placed in an electric alternating induction field which causes heating and melting first of the exterior surface strata of the pattern followed by melting of the deeper parts of the pattern. The melting out and removal of the pattern material from the mold hould be done in an inert atmosphere such as dry CO nitrogen, argon, helium or the like, or under vacuum, in order to prevent oxidation of the mercury thallium compound or its ingredients. After melting and removing the pattern material, the mold is subjected to firing at about 1000 C. as described in said application Ser. No. 440,056, thereby completing the shell mold, which then is ready for casting therein metals of high melting temperature.

This application is a continuatio-n-in-part of my application Serial No. 445,759, filed July 26, 1954, now abanboned.

The features and principles underlying the invention described above in connection with specific exemplifications, will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims shall not be limited to any specific features or details shown and described in connection with the exemplification thereof.

I claim:

1. A disposable casting pattern suitable for forming a shell mold with an investment composition applied to the surfaces of said pattern, which pattern consists of a solid pattern material selected from the group consisting of Hg Tl a mixture of Hg Tl with Hg 'il, and a mixture of Hg Tl with Hg Tl, and in combination with said pattern a protective liquid in contact with the surfaces of said pattern for maintaining said surfaces substantially free of oxygen compounds and with a shiny exterior similar to the shiny exterior surface of solidified frozen pure mer 1 1 cury, and a protective ion exchange agent associated with said liquid for maintaining said liquid substantially free of-thallium ion impurities.

2. The method of preparing a casting pattern suitable for forming over its surface a thin porous shell mold, which method comprises mixing liquid mercury and liquid thallium in amounts corresponding to a mercury thallium material selected from the group consisting of Hg Tl a mixture of Hg Tl with Hg Tl, and a mixture of Hg Tl with Hg Tl under a protective liquid which suppresses oxidation of ingredients of said mixture, treating said liquid for maintaining it substantially free of thallium-ion impurities, and thereafter solidifying the mixed liquids under said protective liquid.

3. The method of preparing a casting pattern as claimed in claim 2, wherein said pattern is formed of separately prepared solidified complementary pattern sections which are brought into contact with each other and jointed into an integral pattern under said protective liquid.

4. A disposable casting pattern suitable for forming a shell mold with an investment composition applied to the surfaces of said pattern, which pattern consists of Hg Tl mixed with Hg Tl in an amount corresponding to a mercury content of at least 27% of the weight of said pattern, and in combination with said pattern a protective liquid in contact with the surfaces of said pattern for maintaining said surfaces substantially free of oxygen compounds and with a shiny exterior similar to the shiny exterior surface of solidified frozen pure mercury, and an ion exchange agent associated with said liquid for maintaining it'substantially free of thallium ion impurities.

5. The combination as claimed in claim 1, said liquid predominantly consisting of ethylene glycol.

6. The combination as claimed in claim 1, said protective liqu'd including an ion exchange agent to-remove from said liquid metallic ion impurities.

7. The combination as claimed in claim 1, said protective liquid including an oxidation inhibitor associated with said liquid to bind oxygen dissolved in said liquid.

8. The combination as claimed in claim 1, said protective liquid including an oxidation inhibitor with said liquid to bind oxygen dissolved in said liquid, and an ion exchange agent for subjecting said liquid to an ion exchange action which removes from said protective liquid metallic ion impurities.

9. The combination as claimed in claim 1, said protective liquid being selected from the group consisting of diatomic alcohols of the aliphatic series, polyatomic alcohols of the aliphatic series, mixtures of said alcohols, and mixtures thereof with water.

10. The combination as claimed in claim 1, said protective liquid being selected from the group consisting of diatomic alcohols of the aliphatic series, polyatomic alcohols 12 of the aliphatic series, mixtures of said alcohols and mixtures thereof with water, said protective liquid including an oxidation inhibitor to bind oxygen dissolved in said protective liquid.

11. The combination as claimed in claim 1, said protective liquid being selected from the group consisting of diatomic alcohols of the aliphatic series, polyatomic alcohols of the aliphatic series, mixtures of said alcohols, and mixtures thereof with water, said protective liquid including an oxidation inhibitor associated with said liquid to bind oxygen dissolved in said liquid, and an ion exchange agent for subjectng said liquid to an ion exchange action which removes from said protective liquid metallic ion impurities.

12. The method as claimed in claim 2, comprising subjecting said protective liquid to an ion exchange action which removes metallic ions from said protective liquid while said pattern is formed under said protective liquid.

13. The method as claimed in claim 2, comprising embodying in said protective liquid an oxygen inhibitor substance operative to bind oxygen dissolved in said protective liquid while said pattern is formed under said protective liquid.

14. The method as claimed in claim 2, comprising embodying in said protective liquid an oxygen inhibitor substance operative to bind oxygen dissolved in said protective liquid and subjecting said protective liquid to an ion exchange action which removes metallic ions from said protective liquid while said pattern is formed under said protective liquid.

15. The method as claimed in claim 2, the protective liquid under which said pattern is formed being selected from the group consisting of diatomic alcohols of the aliphatic series, polyatomic alcohols of the aliphatic series,

mixtures of said alcohols, and mixtures thereof with water.

References Cited in the file of this patent UNITED STATES PATENTS 2,205,854 Kroll June 25, 1940 2,400,831 Kohl May 21, 1946 2,711,570 Sindeband June 28, 1955 FOREIGN PATENTS 689,658 Great Britain Apr. 1, 1953 945,912 France Dec. 6, 1948 OTHER REFERENCES Der Aufbau der Zweistofiiegierungen by Hansen. Pub.

1943, by Edwards Bros, Inc., Ann Arbor, Michigan. Pages 815-819.

Steel: March 19, 1951, pp. 66-69.

American Foundryrnan, February 1955, pp. 39-40. 

1. A DISPOSABLE CASTING PATTERN SUITABLE FOR FORMING A SHELL MOLD WITH AN INVESTMENT COMPOSITION APPLIED TO THE SURFACES OF SAID PATTERN, WHICH PATTERN CONSISTS OF A SOLID PATTERN MATERIAL SELECTED FROM THE GROUP CONSISTING OF HG5TL2, A MIXTURE OF HG5TL2 WITH HG2TL, AND A MIXTURE OF HGG5TL2 WITH HG3TIL, AND IN COMBINATION WITH SAID PATTERN A PROTECTIVE LIQUID IN CONTACT WITH THE SURFACES OF SAID PATTERN FOR MAINTAINING SAID SURFACES SUBSTANTIALLY FREE OF OXYGEN COMPOUNDS AND WITH A SHINY EXTERIOR SIMILAR TO THE SHINY EXTERIOR SURFACE OF SOLIDIFIED FROZEN PURE MERCURY, AND A PROTECTIVE ION EXCHANGE AGENT ASSOCIATED WITH SAID LIQUID FOR MAINTAINING SAID LIQUID SUBSTANTIALLY FREE OF THALLIUM ION IMPURITIES. 